FIG. 1 schematically illustrates the recording medium transport mechanism of a conventional ink jet printer. The conventional ink jet printer includes a first driving roller (which, together with a first driven roller 4, constitutes first transport means) 3 located upstream of a carriage unit 2 having an ink head 7 in a direction in which recording medium 1 is transported and is configured to transport recording medium 1 such as a recording sheet to a location just below an ink jet nozzle 7 of the carriage unit 2 where recording medium 1 is to be temporarily halted, and then move the carriage unit 2 in a direction (primary scanning direction) perpendicular to the recording medium 1 transport direction for printing on the recording medium 1. Thereafter, the ink jet printer advances the recording medium 1 by means of the first driving roller 3 again so that a portion of the recording medium 1 to be subjected to printing next is located just below the ink head 7 and then moves the carriage unit 2 during temporary stop of recording medium 1 for printing. These operations are repeated to form an image on whole one side of recording medium 1.
In order to increase the printing speed of such a recording apparatus without varying the speed of movement of the carriage unit 2 and the recording medium transport speed, the recording medium 1 is moved by the maximum printable width of the ink head 7 for each advance thereof. Thus, the movement of the carriage unit 2 and the advance of the recording medium 1 are repeated. If moving ranges of the ink head 7 are made to overlap each other with the amount of each advance of the recording medium 1 reduced, inks can be jetted onto the same portion of the recording medium 1 in an ink-on-ink manner thereby printing an image of higher definition.
On the downstream side of the carriage unit 2 is disposed a second transport means for transporting and ejecting the recording medium 1, the second transport means comprising a second driving roller 5 to which a rotational force is transmitted, and a star wheel 6 which is driven to rotate while nipping the recording medium 1 between itself and the second driving roller 5. The star wheel 6 forming part of the second transport means has sharp teeth circumferentially thereof and is supported so as to be capable of coming into contact with and separating from the periphery of the second driving roller 5 by means of a non-illustrated spring member. The star wheel 6 biases the recording medium 1 toward the second driving roller 5 by the elastic force of the spring member (see Japanese Patent Laid-Open Publication No. H08-208094 for example.)
However, in the ink jet printer configured to repeat a sequence of advancing and temporarily halting the recording medium 1 and moving the carriage unit 2, it is possible that the recording medium 1 stops being transported when the leading edge of recording medium 1 reaches a location just past the nipping position between the second driving roller 5 and the star wheel 6, i.e., the position where the recording medium 1 receives the transporting force from the second driving roller 5. In this case, since the star wheel 6 is supported so as to be capable of coming into contact with and separating from the periphery of the second driving roller 5, it is possible that the second driving roller 5 and the star wheel 6 fail to ideally catch the recording medium 1 therebetween when the leading edge of the recording medium 1 is not in a good condition. For this reason, the second driving roller 5 and the star wheel 6 cannot reliably transport the recording medium 1, thus raising a problem of unstable transport of recording medium 1.
For example, when the leading edge of recording medium 1 is deformed downwardly as shown in FIG. 2, the leading edge is brought into contact with the second driving roller 5 first and then raised as kept deformed downwardly as the second driving roller 5 rotates, thereby pressing the star wheel 6 upwardly away from the second driving roller 5. When the star wheel 6 becomes separated from the second driving roller 5, the star wheel 6 cannot rotate because the star wheel 6 does not directly receive the rotational force but is driven to rotate by the second driving roller 5. In such a case the star wheel 6 becomes an obstacle to transport of the recording medium 1. Thus, it becomes possible for the star wheel 6 to facilitate deformation of recording medium 1 or damage the recording medium 1 and, in the worst case, to cause jam to occur.
To avoid such a phenomenon, it is conceivable that the star wheel 6 is configured to apply the recording medium 1 with sufficient pressure to rectify the deformed condition of the leading edge of recording medium 1 to an ideal condition. However, if the second driving roller 5 is pressurized excessively by the star wheel 6, large load fluctuations occur when the recording medium 1 advances to the contact position between the second driving roller 5 and the star wheel 6, so that smooth transport of the recording medium 1 is impeded on the contrary. Also, since the periphery of the star wheel 6 comprises a multiplicity of sharp teeth, application of excessive pressure to the recording medium 1 by the star wheel 6 will result in a problem of damage to the surface of recording medium 1.
Such problems are common to recording apparatuses of the type having transport means comprising a roller pair including a star wheel as a driven roller, as well as to ink jet printers.
An object of the present invention is to provide a recording apparatus which is capable of reliably nipping a recording medium by second transport means including a star wheel without the need to apply excessive pressure onto the recording medium from the star wheel even when a leading edge portion of the recording medium has some deformation, thereby constantly ensuring smooth and stable transport of the recording medium.